Temperature compensation in differential pressure leak detection

ABSTRACT

A microprocessor is employed to control a test program which operates to cause a component under test to be filled (or evacuated) to a predetermined pressure P 2  at which a leak test is to be carried out. The test program is arranged firstly to perform a rapid test at atmospheric pressure P 1  to determine the pressure change dpl&#39;/dt in the component due to the temperature fluctuation. The component is then filled (or evacuated) automatically to the test pressure P T , at which the leak check is carried out. Temperature compensation is achieved by subtracting the value of C dpl&#39;/dt from the pressure change dp t  /dt actually measured, i.e. the actual leak rate dPL/dt is given by ##EQU1## where the constant, C, is generated from an identical proram run, the calibration run, on a known non-leaker with a high temperature fluctuation.

BACKGROUND OF THE INVENTION

The present invention is concerned with a method and apparatus fortemperature compensation in differential pressure leak detection.

Many components produced by the manufacturing industry are checked forgas leaks by a method known as "pressure drop". The method entailsfilling the component with a gas (usually air) to the working pressureof the component. The supply is then switched off and the pressure ismonitored. If the pressure falls then a leak is indicated.

The relationship between pressure fall and leakrate is derived fromBoyles law according to which, for a fixed mass of gas:

    ξpv=constant                                            (1)

For a vessel of volume v filled to pressure, p bar, with a leak, Δv,measured at atmospheric pressure, (1 bar), then from Boyles law,

    pv=(p+Δp)v+1Δv                                 (2)

where Δp is the pressure change. Then

    0=Δpv+Δv                                       (3)

The rate of pressure change dp/dt is given by ##EQU2## where dv/dt isthe volumetric flowrate measured at atmosphere pressure 1(bar)

Thus, the rate of fall of pressure can be used as a measure of thevolumetric leak from the component.

Unfortunately, however, temperature fluctuations have a similar effecton pressure within the vessel. Thus, for a mole of gas, the gas lawsstate that

    pv=Rθ                                                (5)

where R is the gas constant and θ is the temperature. In changingconditions, ##EQU3##

If the rate of change of volume of the component is ignored, then to afirst approximation ##EQU4## substituting from (5) gives ##EQU5##

For any fixed mass of gas with only very small temperature fluctuationduring the test, then equation (8) to a first approximation becomes.##EQU6##

In practice, the relationship between rate of change of pressure due toa fixed rate of change of temperature and the actual pressure in thevessel can be more nearly described by the second order equation:##EQU7## where a and K are constants.

In a real test situation, it is therefore impossible to tell whether ameasured pressure change is due to a small leak or to a smalltemperature fluctuation, or both.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method by whichcompensation for temperature fluctuations can be achieved so as toobtain an accurate measure of actual leakrate.

As stated above, in a real test situation, it is not possible to tellwhether the measured pressure change is due to a small leak or to asmall temperature fluctuation. Temperature changes as small as one tenthousandth of a degree Kelvin will affect the measurement. However, whenthe vessel is filled with air to atmospheric pressure and sealed, thenany pressure fluctuation occurring is entirely due to temperaturefluctuation since no leak will be present.

In a crude system it would be sufficient to measure the pressurefluctuation in a pre-test at atmospheric pressure (1 bar) thenextrapolate this to the filling pressure, P₂ bar in equation (9). i.e.at 1 bar ##EQU8##

At P₂ bar, the predicted pressure fluctuation due to temperature changewould be: ##EQU9##

Suppose however, that the pressure changes due to a combination oftemperature change and leakrate, then the leak contribution, dp_(L), /dtis given by ##EQU10## where dp/dt is the measured pressure differential.

Thus, in the crude system temperature compensation could be achieved byinserting the results of equation (11) in (12). ##EQU11##

A more accurate compensation is achieved, however, by employing the moreaccurate relationship described by equation (10). In this case, a known,non-leaking vessel subjected to a significant rate of change oftemperature must first be tested. Using a non-leaker, a test is firstmade at 1 bar absolute followed by a test at the required test pressurep₂. Then, at p=1, equation (10) gives ##EQU12## and at P₂, ##EQU13##From these two equations, the constant `a` can be shown to be given by##EQU14##

If an unknown component is first tested at atmospheric pressure, themeasured rate of change of pressure dp_(1') /dt relates to a newtemperature rate of change dθ'/dt as described in equation (14)##EQU15## and similarly at pressure P₂, ##EQU16## Substituting forKd0'/dt from (17) ##EQU17## Substituting for `a` from equation (16)gives: ##EQU18##

The measured values of dp¹ /dt and dp₂ /dt can then be used to computeC, so that ##EQU19## (this expression holding true only at the twopressures p₁ and p₂). Substituting in equation (12) gives ##EQU20##where ^(d) PL/dt is the required measure of actual leakrate

dp/dt is the measured pressure change at the test pressure P₂.

C is calculated from the ratio ##EQU21## and dp1'/dt is the measuredpressure change at atmospheric pressure.

Thus, in accordance with the present invention, there is provided amethod of differential pressure leak detection comprising the steps of:

(a) measuring and storing the rates of change of pressure in a knownnon-leaking vessel at an elevated or depressed and changing temperaturefrom ambient, both at atmospheric pressure P₁ to obtain dp₁ /dt and at atest pressure P₂ to obtain dp₂ /dt;

(b) computing the ratio ##EQU22## using the stored values of dp₂ /dt anddp₁ /dt;

(c) measuring the rate of change of pressure (dP_(1') /dt) atatmospheric pressure in an unknown vessel under test;

(d) measuring the rate of change of pressure (dP_(T) /dt) in the testvessel at the test pressure P_(T) ; and

(e) subtracting from the measured pressure change (dP_(T) /dt) theresult of multiplying the measured value of (dp_(1') /dt) from (c) bythe computed value of C from (b).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of the computing means of the subjectinvention incorporated into a controlling system of a known pressuremeasurement means in a first operational condition.

FIG. 2 is a schematic illustration similar to FIG. 1 but showing thesystem in a second operational condition.

FIG. 3 is a flow diagram illustrating one practical embodiment of themethod of the subject invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In an illustrated embodiment of the invention, a microprocessor isemployed to control the test programme and operates to cause thecomponent under test to be filled (or evacuated) to a predeterminedpressure P_(T) at which the leak test is to be carried out. The testprogramme is preferably arranged firstly to perform a rapid test atatmospheric pressure P₁ to determined the pressure change dp1'/dt due tothe temperature fluctuation. The component is then filled (or evacuated)automatically to the test pressure P_(T), at which the leak check iscarried out. The apparatus used for filling the components, andmeasuring the pressure change rates is itself conventional.

More particularly, FIGS. 1 and 2 show a typical known pressuremeasurement system that may be employed with a microprocessor or othercomputing means to practice the subject invention. In this regard, FIGS.1 and 2 show a leakage inspection system which is a simplification of asystem identified as prior art in U.S. Pat. No. 4,675,834. This system,as depicted in FIGS. 1 and 2 is identified generally by the numeral 20and includes a pressure generator 22 which is selectively placed incommunication with either a vessel 24 under test or a calibration vessel25. A valve means 26 is disposed intermediate the pressure generator 22and the vessel under test 24 or the calibration vessel 25 forselectively sealing the appropriate vessel 24, 25. A pressure sensingmeans 28 is in communication with the vessel under test 24 or thecalibration vessel 25 intermediate the valve means 26 and theappropriate vessel 24 or 25. The microprocessor or other computing meansin accordance with the subject invention is identified generally by thenumeral 30 and is operatively connected to the pressure sensing means 28and the valve means 26 for generating and storing signals correspondingto rates of change of pressure in the vessel under test 24 as explainedabove and further herein. FIG. 1 shows the calibration vessel 25connected to the system 20 for performing a calibration of the system20. In FIG. 1, the vessels 24 to be tested are in an off line location.FIG. 2 shows one of the vessels 24 connected to the system 20 fortesting. In FIG. 2, the calibration vessel 25 is in an off linelocation, but remains available for periodic calibration of the system.The operation of the microprocessor or computing means 30 is describedin greater detail below and is illustrated schematically in the flowdiagram of FIG. 3. Temperature compensation is achieved by themicroprocessor subtracting the result of the calculation given inequation (21) above from the pressure change dP_(T) /dt actuallymeasured, i.e. the actual leak rate dpL/dt is given by ##EQU23##

The constant, C, is generated from an identical programme run, thecalibration run, on a known non-leaker with a high temperaturefluctuation. Thus, all programme times and pressure are identicalbetween the calibration and normal test runs.

Referring to the illustrated flow diagram, the steps controlled by themicrocomputer are as follows, the following step numbers correspondingto the numbers given to each of the respective operational steps in theflow diagram=

(1) A test run is initalled by operating a "start" control.

(2) If a "calibration run" is to be performed, the test proceeds to step(3). If a test run on a test component is to be performed, the testproceeds to step (11), (see below).

(3) If a non-leaker component has been connected and a desiredtemperature condition prevails, then the calibration run proceeds tostep (4).

(4) The system is sealed at atmospheric pressure and temperature isaltered from ambient.

(5) A measure is made of the pressure change PC₁ over time T_(A).

(6) dP₁ /dt=PC₁ /T_(A) is obtained and stored.

(7) The system is filled (or evacuated) to a predetermined test pressureP₂.

(8) A measure is made of the pressure change PC₂ over time T_(B)(normally again at the said desired temperature).

(9) dP₂ /dt=PC₂ /T_(B) is obtained and stored.

(10) The ratio ##EQU24## is calculated from the stored values from steps(6) and (9) in order to calculate C, which is then stored.

(11) If a test component has been connected, then the test run proceedsto step (12).

(12) The system is sealed at atmospheric pressure.

(13) A measure is made of the pressure change PC₁ ' over time T_(A).

(14) dP_(1') /dt=PC_(1') /T_(A) is obtained and stored.

(15) The system is filled (or evacuated) to a predetermined testpressure P_(T).

(16) A measure is made of the pressure change PC over time T_(B)(normally at the same temperature as step (13).

(17) dp_(T) /dt=PC/T_(B) is obtained and stored.

(18) The value of C from step (10) is multiplied by dp_(1') /dt fromstep (14) and subtracted from dp_(T) /dt from step (17) to compute thecorrected leak rate result dpL/dt from: ##EQU25##

It will be evident that the described method relies on the fact that thetemperature of the test and calibration components is changing duringmeasurement and thus giving rise to effective pressure changes. However,since the calculations involve taking ratios involving the rate ofchange of temperature, then the actual value of the rate of change oftemperature does not matter, so long as it is substantially the same forthe two test measurements, i.e. at one bar and at the test pressure.

Hence, it is necessary to take the pressure readings as quickly aspossible, since most components obey Newton's law of cooling, i.e. anexponential change and therefore one having a varying rate of change.

In practice, it does not matter whether the component under test isheating or cooling, but its temperature must be changing. The actualtemperature and the rate of change of temperature can be different forthe calibration and the test run, as will be appreciated from theequations.

Since the effects of temperature can be small, in order to achieve goodaccuracy in the calculations it is preferred to perform the calibrationrun on as high a temperature as possible (or as low) since the rate ofchange is then highest. This reduces the error in the constant C whencomputed by a digital system.

In practice the calibration component could be deliberately raised orlowered in temperature (for example by an oven or fridge) and thenallowed to cool/warm, but typically the test components are alreadyhot/cold (and therefore cooling/warming) due to the environment of themanufacturing process through which they have been.

Thus, the present method enables accurate compensation to be made inleak tests for the effects of temperature variations on the measuredpressure changes.

I claim:
 1. A method for accurately determining the actual leak ratefrom a test vessel of unknown leaking characteristics, said methodcomprising the steps of:providing a calibration vessel known to besubstantially free of leaks; filling the calibration vessel with air toprevailing atmospheric pressure, P₁ ; altering the temperature in thecalibration vessel from ambient temperature; sealing the calibrationvessel; generating and storing a signal corresponding to the rate ofchange of pressure in the calibration vessel to determine dP₁ /dt;altering the volume of air in the calibration vessel to achieve a testpressure P₂, which is different from P₁ ; sealing the calibrationvessel; generating and storing a signal corresponding to the rate ofchange of pressure in the calibration vessel to determine (dP₂ /dt);computing a ratio, C, of (dP₂ /dt) divided by (dP₁ /dt); filling thetest vessel with air to prevailing atmospheric pressure P₁ ; sealing thetest vessel; generating and storing a signal corresponding to the rateof change of pressure in the test vessel to determine (dP₁ /dt)';altering the volume of air in the test vessel to achieve the testpressure P_(T) ; generating and storing a signal corresponding to therate of change of pressure in the test vessel to determine (dP_(T) /dt);and generating a signal corresponding to the actual leak rate from thetest vessel by subtracting from the measured rate of pressure change(dP_(T) /dt) the product of the ratio C multiplied by the measured rateof pressure change (dP₁ /dt)'.
 2. A method as in claim 1 for accuratelydetermining the actual leak rate from a plurality of test vesselswherein the steps of filling the test vessel with air to prevailingatmospheric pressure, sealing the test vessel, generating and storing asignal corresponding to the rate of change of pressure in the testvessel to determine (DP₁ /Dt)', altering the volume of air in the testvessel to achieve a test pressure P_(T), and generating and storing asignal corresponding to the rate of change of pressure in the testvessel to determine (DP_(T) /Dt)' are repeated sequentially for each ofsaid test vessels and wherein the method further includes the steps ofgenerating signals corresponding to the actual leak rate from each ofthe test vessels by subtracting from the measured rate of pressurechange (DP_(T) /Dt)' for each of said test vessels the product of theratio C multiplied by the measured rate of pressure change (DP₁ /Dt)'.3. A method as in claim 1 wherein the step of altering the volume of airin the test vessel is carried out automatically after the step ofgenerating and storing a signal corresponding to the rate of change ofpressure in the test vessel to determine (DP₁ /Dt)'.
 4. A method as inclaim 1 wherein the test pressure P₂ is approximately equal to the testpressure P_(T).